A current reference generator is a useful component for providing a known current level within an electronic circuit. Classic current reference generators were typically implemented using bipolar transistors and resistors and the like. Many modern electronic devices, however, are implemented using complementary metal-oxide semiconductor (CMOS) technology for reduced size and power consumption. CMOS technology, for example, is particularly advantageous for smaller and/or lower power electronic devices including those which are powered by a battery. Although bipolar devices may be integrated along with CMOS devices on a common chip (e.g., BiCMOS or BiMOS), it is preferred to implement as many devices components as possible using the same process because it is generally easier, less expensive, and more efficient. Resistors can also be problematic since they generally consume a significant amount of space, and precision resistors are relatively difficult to fabricate.
U.S. Pat. No. 5,949,278, entitled “Reference Current Generator In CMOS Technology,” invented by Henri Oguey taught an implementation of a CMOS current reference generator without the use of bipolar devices or resistors. The CMOS current reference generator described in this patent exhibited insensitivity to temperature and process variations. The primary configuration described therein, however, also exhibited an undesirable degree of dependence upon the supply voltage. An additional configuration was described which added a cascode stage to reduce the dependence upon supply voltage variations. The added series-coupled cascode stage, however, increased the minimum supply voltage level sufficient to properly operate the current generator. In particular, the added cascode stage raised the minimum supply voltage to about twice the threshold voltage (VT) of the individual MOS transistors, or to about 2VT. Although MOS devices do not completely shut down when their gate-to-source voltage (VGS) is below VT for sub-threshold operation, the current becomes very low during sub-threshold operation so that VT is a practical limitation for reliable operation. The VT of most readily available technologies is above 0.7 Volts (V), so that the current generator described by the Oguey patent implemented using these technologies required a supply voltage of well over 1V (e.g., 1.4-1.8V). Certain newer technologies have reduced VT of the devices to about 0.4V. Nonetheless, the requirement that the current generator operate with a supply voltage of about 2VT prevents the full benefits of the lower voltage technologies.
The higher voltage level is not advantageous for certain applications, such as battery-operated devices with limited supply voltage range. It is desired to provide a current reference generator that is independent of supply voltage and which successfully operates using a supply voltage of at the threshold voltage VT of the applicable technology.